Biomass: Digestor/Reactor vs Gasifier Efficiency

Question

When using biomass for energy, two popular options are reactors and gasifiers. The first uses anaerobic micro organisms to convert the biomass to fertilizer and a methane rich gas mixture. The second uses heat to pyrolyze the biomass into ash and a carbon monoxide and hyrdogen rich gas mixture.

In terms of the overall potential energy produced by their resulting gas mixtures, is there an efficiency difference between the two?

For example, if you left one unit of wood in a digestor/reactor and let it sit until it was completely consumed by the system would the resulting amount of methane rich gas have more potential energy than if we ran that same unit of wood through a gasifier and captured the resulting amount of carbon monoxide and hyrdogen rich gas.

Answer 1

Your comparing apples to oranges, or rather apple fruits to apple trees. AD (Anaerobic digestion) can only use parts of the plants that a cow could eat - starch, fat, protein and some fibre (hemicellulose) but not woods biomass. Pyrolysis OTOH can, and is most often done, on wood.

In theory pyrolysis is exothermal in certain temperature ranges, AFAIK the heat is usually supplied by burning a small amount of the biomass with air - maybe 10% of the energy content.

AD is exothermal in the sense that it generates a small amount of heat (a few percent of the energy content of the biogas), in most practical cases this is negligible compared to the energy needed to heat the substrate to 40°C required for most fermentation processes.

Both processes require electrical energy for handling (or not, some small digesters can be stirred by hand). Again, the comparison is difficult. Pyrolysis is usually (the exception is hydrothermal carbonization which is very cool but a different beast) done on dry matter, AD on wet slurries. So the energy density (usable energy / kg) is far lower for most AD feedstocks.

Lastly, treating biogas is far easier than treating pyrolysis gas. In every case, the burner needs to be adjusted for the Wobbe Index of the gas. With biogas, desulphurization and drying are the most common steps before use. With pyrolysis, you need to remove dust and tar, also cooling is often required (to achieve better density).

Answer 2

Generally speaking, there is an efficiency difference related to the latent heat of the moisture of the feedstock. Moisture in the feedstock needs to be evaporated before thermal degradation occurs; however, biological degradation does not require the moisture to be evaporated. Because of the need to evaporate moisture gasifiers, in general, tend to be less efficient than anaerobic digesters.

There is research into hydrothermal gasification which overcomes this issue by gasifying materials underwater at supercritical temperatures.